The invention concerns a monodisperse stable double emulsion of the water-in-oil-in-water type, its method of preparation and the polydisperse stable double emulsion used as a starting emulsion in the preparation method.
The advantage of double emulsions is widely recognised in fields as diverse as pharmaceuticals, cosmetics, pest and disease control, food and/or coatings of the paint type.
The double emulsions of the water-in-oil-in-water type allow in particular the encapsulation of various active substances at the internal aqueous phase. Under well determined conditions, it is in fact possible to cause the salting out of the encapsulated active substances whilst controlling their release kinetics.
The preparation of stable double emulsions of the water-in-oil-in-water type comprising an active substance in their internal aqueous phase is problematic. The development of such emulsions is generally achieved by trial and error on the basis of empirical rules according to the constituents present in the different phases. In the art, monodisperse double emulsions are particularly sought because of their homogeneity: in particular they allow a regular salting out of the active materials.
Various methods of preparing monodisperse emulsions are known: a first method is that described in EP 442 831 and EP 517 987.
This method involves the fractionation of a starting polydisperse primary emulsion, by successive creamings. It is lengthy and tedious and not easily applicable on an industrial scale. A second method is described in FR 97 00 690 or in International Application WO 97/38787. It consists of subjecting a starting viscoelastic primary emulsion to a controlled shearing so that the same maximum shearing is applied to all the emulsion. This method has various advantages and in particular allows control over the size of the droplets of the monodisperse emulsion obtained.
When one or other of these methods is applied to a double emulsion, it is essential not to introduce the destruction of the double emulsion by causing for example the coalescence of the droplets forming the emulsion or the premature leakage of the active principle.
Under these circumstances, it will be understood that the development of a double emulsion of the water-in-oil-in-water type which is both stable and fractionatable is extremely tricky. It must be understood that, in the context of the invention, fractionatable emulsion means an emulsion which it is possible to process according to the methods described above with a view to obtaining an emulsion, of the same nature (of the water-in-oil-in-water type), with the same formulation (same compositions of the different phases), and which is both stable and monodisperse.